Adhesive compositions are used to affix (bond) glass (windows) into buildings and vehicles, see Rizk, U.S. Pat. No. 4,780,520; Bhat, U.S. Pat. No. 5,976,305; Hsieh et al, U.S. Pat. No. 6,015,475 and Zhou, U.S. Pat. No. 6,709,539. In automobile factories, windows are installed using robots and computer controlled processing which facilitates the use of a variety of high performance adhesives. These adhesives have to be storage stable and be readily applied under high shear, for example, to a windshield, while retaining a bead shape so that the windshield can be properly fitted to the automobile. This has been achieved by one part moisture curable polyurethane prepolymer based adhesives having fillers, prepolymers having sufficient molecular weight and, in particular, plasticizers, so that they are still pumpable and can be applied to the windshield without stringing, sagging while also quickly obtaining and retaining the sufficient strength and elasticity (i.e., low glass transition temperature Tg) required of an adhesive over the large range of environmental conditions encountered by an automobile.
The ability to apply high viscosity adhesives via pumps imparting high shear rates and maintain good bead geometry or shape and while holding heavy components in place without the use of mechanical fixtures such as tape or clips or clamps has required plasticizers. This rheological behavior has required the use of high boiling point (e.g., boiling point greater than about 85° C.) solvents, referred to as plasticizers. Unfortunately, plasticizers such as phthalate have caused health concerns such as possibly being a carcinogen and recently have come under further regulation. Likewise, plasticizers are known to migrate from adhesives over time and volatilize degrading the adhesive and causing fogging of the windows in the vehicle.
What is needed is a prepolymer and adhesive composition for bonding glass into a structure which may be formulated, with little or no plasticizer while still exhibiting multiple desirable characteristics including: low viscosity, low isocyanate content, high yield value and recovery after shear, low glass transition temperature (Tg) even after exposure to accelerated weathering and over time which can result in phase separation of plasticizer from the matrix. In addition, the prepolymer should cure quickly after exposure to moisture with sufficient strength, elongation and Young's Modulus to have the windshield act as a structural element of the vehicle. It should also display quick buildup of strength and modulus to allow for fast safe drive away times when applied under a variety of conditions, while not displaying sag or string when applied even after undergoing high shear just prior to being applied.
Sag is the loss of the shape of the adhesive bead, often as the result of gravitational forces. If severe enough, this deformation can interfere in the proper installation and sealing of the window into the vehicle. Stringing of an adhesive is the formation of a long string of adhesive at the end of the bead of adhesive dispensed which can complicate application of the adhesive and cause imperfections in the installed adhesive bead.